Narrowband (311-313 nm) ultraviolet B phototherapy with the Philips TL-01 lamp is used increasingly in the treatment of psoriasis with little information available on the optimum irradiation regimen. We compared a high and a low incremental dose regimen in 20 patients with symmetrical chronic plaque psoriasis using a randomized half body study and thrice weekly exposures. Paired trunk, leg and arm plaques of psoriasis were scored blind prior to and at each treatment for scaling, erythema and induration. Patients were treated to clearance or minimal residual activity and followed up until relapse. The low increment regimen achieved a 10% reduction in the median cumulative dose to clearance (16,401 vs. 18,246 mJ/cm2) with one extra treatment in 50% of the patients. However, the duration of treatment (median 53.5 days) was identical for both regimens except for one patient because there were 50% fewer episodes of erythema requiring postponement of treatment with the low increment regimen. We now favour the low increment regimen for phototherapy in our psoriasis population.